Polypropylene resins have various uses such as films, fibers and other molded articles of various shapes, since they are inexpensive and molded articles thereof are superior in rigidity, heat resistance, chemical resistance and electrical insulating property.
However, since conventional polypropylene resins are crystalline resins, the melt viscosity, particularly the elongational viscosity, is very low. Thus, they have the problem that in a process wherein a molten resin is remarkably deformed, e.g., blow molding, extrusion, and foaming, ranges of processing conditions capable of producing suitable molded articles are narrow.
As one of manners for solving such a problem in processability of polypropylene resins, physical blending of the polypropylene resins with polyethylene has been widely used. However, according to this manner, a large amount of polyethylene is required for sufficiently improving the processability, so the molded articles obtained from the thus improved resins result in low rigidity.
Another manner is to use a polypropylene resin having a high molecular weight. However, such a resin has the problem that in case of conducting extrusion, a load on an extruder becomes large because of high viscosity. Also, since the resin is difficult to largely deform in the molten state, blow molding thereof is difficult to provide molded articles in a high draw ratio and foaming thereof is difficult to provide foamed articles having a high expansion ratio.
Also, polypropylene resins are radical-degradable polymers. Therefore, in general, it is difficult to increase the molecular weight by crosslinking the polymer molecules, even if radical polymerization initiators capable of serving as an intermolecular crosslinking agent are used.
In order to solve these problems, it is proposed in Japanese Patent Publication Kokai No. 62-121704 to irradiate radiation to a semi-crystalline polypropylene resin having a linear molecular structure in a gas atmosphere having a controlled specific oxygen concentration, thereby raising the melt viscosity and melt tensile strength of the polypropylene resin. The Japanese publication discloses that the polypropylene resin modified in this manner has a structure such that the polymer molecule has long chain branches, and is suitable for molding methods such as blow molding, extrusion coating molding and the like.
It is also disclosed therein that the reason why the polypropylene resin modified in this manner is suitable for these molding methods is that the resin has the characteristics that the elongational viscosity measured in the molten state of this resin rapidly increases as the strain increases, that is to say, when this resin is subjected to tensile deformation in the molten state, the elongational viscosity increases with increasing the elongational strain and, after reaching a specific elongational strain, the elongational viscosity rapidly increases.
However, in case of modifying polypropylene in this manner, it is unavoidable that because of utilizing radiation, the apparatus to be used becomes large scale and complicated in structure. Also, in order to prevent degradation and gelation of polypropylene resins in the radiation exposure step so as to stably produce products, it is required to strictly control the radiation dose and the oxygen concentration of the surrounding gas. Further, because of necessity to strictly control the production conditions like this, it is not easy to give variety to the physical properties of the objective modified polypropylene resins.
Foamed articles made of polypropylene resins have good chemical resistance, impact resistance and heat resistance and, therefore, they have been suitably utilized as heat insulator, cushioning material, core material, food container and the like, particularly as cushioning material.
However, polypropylene resins are low in melt viscosity and melt tensile strength because of crystalline polymers as stated above and, therefore, in case of foaming the resins, the strength of cell wall is not sufficiently retained at the time of foaming. Also, the polypropylene resins are poor in gas barrier property and, therefore, a gas generated from a blowing agent is easy to escape from the cells to the outside when foaming the resins. For such reasons, it has been difficult to obtain foamed articles having an excellent appearance, a high closed cell content and a low density from polypropylene resins.
As a manner of improving the foamability of polypropylene resins, there are known, for example, a process wherein a crosslinking assistant is added to a polypropylene resin to crosslink the molecules thereof (see for example Japanese Patent Publication Kokoku No. 45-40420) and a process wherein a polypropylene resin is blended with polyethylene and is foamed (see for example Japanese Patent Publication Kokoku No. 44-2574).
However, the improvement of the foamability is not sufficient even if these processes are used.
Also, Japanese Patent Publication Kokai No. 4-3642 discloses a process wherein a propylene polymer and a non-conjugated diene are melt-blended to give a propylene-nonconjugated diene copolymer, and the copolymer is foamed. However, the Japanese publication does not disclose any process using isoprene.
Like this, it is the present situation that a process for preparing foamed articles having a high expansion ratio, a high closed cell content and a low density by sufficiently improving the foamability of a polypropylene resin and using the improved resin.
It is an object of the present invention to provide a modified polypropylene resin having a high melt viscosity, particularly a high elongational viscosity in the molten state, by a simple apparatus and in a simple manner.
A further object of the present invention is to provide a modified polypropylene resin which has the characteristics that the elongational viscosity thereof measured in the molten state rapidly increases as the amount of strain increases and which is excellent in extrusion moldability, foamability, blow moldability and vacuum moldability.
Another object of the present invention is to provide a foam which is made of a modified polypropylene resin having an improved foamability and which has an excellent heat resistance, a high expansion ratio, a high closed cell content, a low density and a beautiful appearance.
The present inventors have made an intensive study in order to solve the above-mentioned problems and have now found that a modified polypropylene resin obtained by melt-kneading a polypropylene resin, an isoprene monomer and a radical polymerization initiator has a melt viscosity not lowered in excess and favorable physical properties that the starting polypropylene resin originally possesses without being impaired, and has a high elongational viscosity at melting and accordingly has a drastically improved processability.
It has also been found that a modified polypropylene resin obtained by kneading a polypropylene resin, an isoprene monomer and a radical polymerization initiator at a temperature at which the polypropylene resin is molten and the radical polymerization initiator is degradable, has a remarkably improved foamability as compared with the starting polypropylene resin prior to the melt-kneading and has an excellent heat resistance, so it provides foamed articles having an excellent heat resistance, a high closed cell content, a low density and an excellent appearance by foaming it. Thus, the present inventors has accomplished the present invention.